System and method for early detection of contaminants in a fuel processing system

ABSTRACT

A system and method for preventing damage to a fuel cell stack resulting from impurities in the product stream from a fuel processor, such as may occur should the separation region of the fuel processor fail. The system and method include detecting the concentration of at least one component of the product stream and isolating the fuel cell stack should this concentration exceed an acceptable threshold level. In some embodiments, the system or method are adapted to detect the concentration of a component that itself is not harmful to the fuel cell stack, or which is not harmful in an associated threshold concentration. In some embodiments, the detected composition is at least one of water, methane, and carbon dioxide.

RELATED APPLICATIONS

This application is a divisional patent application of and claimspriority to U.S. patent application Ser. No. 10/244,904, which was filedon Sep. 16, 2004, issued on Nov. 16, 2004 as U.S. Pat. No. 6,818,335,and which is a continuation of U.S. patent application Ser. No.09/477,128, which was filed on Jan. 3, 2000, and issued on Sep. 17, 2002as U.S. Pat. No. 6,451,464. The complete disclosures of theabove-identified patent applications are hereby incorporated byreference for all purposes.

FIELD OF THE INVENTION

The present invention relates generally to fuel processing systems, andmore particularly to a system and method for early detection ofimpending failure of hydrogen-purifiers used in such fuel processingsystems.

BACKGROUND AND SUMMARY OF THE INVENTION

Fuel processing systems include a fuel processor, or hydrogen-generatingassembly, which produces hydrogen gas, and a fuel cell stack, whichproduces an electric current and water from the hydrogen gas and air.Because fuel cells are extremely sensitive to certain contaminants, careis taken to prevent the hydrogen feed to the fuel cell stack fromcontaining more then acceptable levels of these contaminants. Thereforethere is a need to detect contaminants in the product hydrogen streamfrom a fuel processor before the contaminated product stream reaches thefuel cell stack.

The present invention provides a system and method for detectingimpurities in the hydrogen product stream of a fuel processing system insufficient time to prevent the impurities from reaching the fuel cellstack associated with the fuel processor. In some embodiments, thesystem or method are adapted to detect the concentration of a componentthat itself is not harmful to the fuel cell stack, or which is notharmful in an associated threshold concentration. In some embodiments,the detected composition is at least one of water, methane, and carbondioxide.

Many other features of the present invention will become manifest tothose versed in the art upon making reference to the detaileddescription which follows and the accompanying sheets of drawings inwhich preferred embodiments incorporating the principles of thisinvention are disclosed as illustrative examples is only.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a fuel processing system according tothe present invention.

FIG. 2 is the diagram of FIG. 1 showing another embodiment of a fuelprocessing system according to the present invention.

FIG. 3 is a flow diagram illustrating a method for early detection ofcontaminants according to the present invention.

FIG. 4 is a schematic front elevation view of a user interface for afuel processing system according to the present invention.

DETAILED DESCRIPTION AND BEST MODE OF THE INVENTION

A fuel processing system is shown in FIG. 1 and indicated generally at10. System 10 includes a fuel processor 12 and a fuel cell stack 14.Fuel cell stack 14 includes at least one fuel cell, and typicallyincludes multiple fuel cells coupled together. The fuel cell stackreceives hydrogen gas from the fuel processor and produces an electriccurrent therefrom as the hydrogen gas is reacted with oxygen to formwater. The electric current produced by the fuel cell stack is then usedto meet the electric load applied by one or more associated devices,such as vehicles, households, generators, boats, etc. Examples ofsuitable fuel cells include proton exchange membrane (PEM) fuel cellsand alkaline fuel cells.

Fuel processor 12 is a device or assembly of devices adapted to producehydrogen gas through any suitable mechanism from a single or multi-component feedstock comprising one or more feed streams. Examples ofsuitable fuel processors include steam reformers. Examples of suitablemechanisms include steam reforming, partial oxidation, autothermalreforming and pyrolysis of a hydrocarbon or an alcohol, and electrolysisof water. It should be understood that the feedstock for fuel processor12 will vary depending upon the particular form of fuel processor beingused. For example, when fuel processor 12 is a steam reformer, thefeedstock will typically include water and an alcohol or hydrocarbon.Autothermal reforming will also include a water component or stream as apart of the feedstock, however, pyrolysis and partial oxidation willnot.

For purposes of illustration, the following description will describefuel processor 12 and its various embodiments generally, since it iswithin the scope of the present invention that fuel processor 12 may beimplemented as any device or series of devices through which hydrogengas is produced, as well as in the context of a steam reformer.

In FIG. 1, a feed assembly 16 is shown delivering the feedstock andwater streams 18 and 20 to fuel processor 12. Feed assembly 16 includesone or more pumps that draw the streams from supplies (not shown). Whenthe feedstock is miscible with water, such as methanol, the feedassembly may include a mixing chamber in which the feedstock and waterare premixed prior to delivery to the fuel processor. Streams 18 and 20may also be preheated, or even vaporized, prior to delivery to the fuelprocessor. When a water stream is not required for the particularmechanism through which fuel processor 12 produces hydrogen gas, then itshould be understood that feed assembly 16 will not include water stream20.

Fuel processor 12 includes a hydrogen-producing region 24, in whichhydrogen gas is produced. This initial hydrogen stream 26 typicallyincludes impurities that must be removed before the produced hydrogengas can be delivered to fuel cell stack 16. In the context of a steamreformer, this region may be referred to as a reforming region, in whichthe feedstock and water are reacted catalytically to produce a hydrogenstream (or reformate stream) containing hydrogen and byproducts.Byproducts of the reforming reaction include carbon dioxide and carbonmonoxide.

Stream 26 passes to a separation region, or separation assembly, 30 inwhich hydrogen gas is at least partially purified to produce a producthydrogen stream 34 that contains at least a substantial portion of thehydrogen gas in stream 26. Separation region 30 removes at least aportion of the impurities from stream 26, resulting in product hydrogenstream 34 having a higher concentration of hydrogen gas than stream 26.Region 30 may retain or consume the removed portion of stream 26, or itmay exhaust the removed portion as a byproduct stream 36, such as shownin FIGS. 1 and 2. One suitable form of separation region includes one ormore hydrogen-selective membranes 32 that permit hydrogen gas to passtherethrough to produce product stream 34, with the remaining componentsof stream 26 that are unable to pass through the membrane(s) formingbyproduct stream 36. Byproduct stream 36 typically will containunreacted feedstock, some hydrogen gas, carbon dioxide, and carbonmonoxide. Suitable hydrogen-selective membranes include membranes formedfrom palladium, palladium alloys or ceramics. Separation region 30 mayinclude any other suitable device or utilize any other suitablemechanism for purifying hydrogen, such as an absorbent bed or acatalytic reactor. Examples of absorbent beds include zeolite and carbonbeds. Examples of catalytic reactors include water-gas-shift reactorsand selective oxidation reactors.

Sometimes the product stream contains compositions other than hydrogengas. This may occur, for example, when there is a small leak or otherdefect in the membrane(s) in the separation region. Therefore, theproduct stream may also contain some concentrations of carbon dioxideand carbon monoxide. Therefore, it is preferable that fuel processor 12includes a polishing region that reduces the concentration ofcompositions that are harmful to fuel cell stack 14 to below acceptablelevels. In FIG. 2, fuel processing system 10 is shown including a fuelprocessor with such a polishing region 38. In polishing region 38, theconcentration of components other than hydrogen in the product streamare reduced, with the specific goal of reducing the concentrations ofcomponents that are harmful to fuel cell stack 14 to below acceptablelevels.

For example, region 38 may include one or more methanation catalyst beds40 that convert carbon monoxide and carbon dioxide in the product streaminto methane and water according to the following reactions:CO₂+4H₂=CH₄+2H₂OCO+3H₂=CH₄+H₂OBecause methane and water will not damage fuel cell stack 14, thepolished product stream 42 will not impair the operation of stack 14, solong as the concentrations of carbon monoxide and carbon dioxide arebelow acceptable minimum levels, as discussed in more detail below.

Sometimes product stream 34 also contains some concentration ofunreacted feedstock and water that also pass through separation region30. Because of this, polishing region 38 may further include a reformingregion 44 to convert any unreacted feedstock into hydrogen gas, carbondioxide and carbon monoxide. This increases the amount of hydrogen gasproduced by the fuel processor, thereby improving the efficiency of thefuel processor. However, it can be understood that the reforming regionof purification region 38 should be upstream from the methanation bed(s)so that any carbon monoxide and carbon dioxide produced in the secondreforming region may be converted into compositions that will not damagethe fuel cell stack.

An example of a suitable steam reforming fuel processor with a polishingregion is disclosed in copending U.S. patent application Ser. No.09/291,447, the disclosure of which is hereby incorporated by reference.It should be understood that a second reforming region is not essentialto the present invention, and that the polishing region may utilize anyother suitable method for removing or reducing the concentration ofcomponents that are harmful to fuel cell stack 14. It should also beunderstood that the present invention may be implemented with fuelprocessors that lack a polishing region.

Regardless of the specific construction of the fuel processor, theseparation region may suffer a failure, namely, a significant loss inhydrogen selectivity. For example, in membrane-based purificationregions, this may occur if there is a physical defect in ahydrogen-separation membrane or if a tear or other hole was introducedto the membrane during operation. Should such a failure occur, theconcentrations of carbon dioxide and carbon monoxide in the productstream will increase dramatically. Fuel processors including a polishingregion will be able to reduce these concentrations to some degree,however, the concentration of carbon monoxide and carbon dioxide islikely to exceed the capacity of polishing region 38.

Left unchecked, the product stream passing to the fuel cell stack wouldcontain concentrations of carbon dioxide and carbon monoxide that exceedthe acceptable minimum levels. Especially of concern is theconcentration of carbon monoxide, because a concentration of carbonmonoxide as low as a few parts per million may poison, or permanentlydamage the fuel cell stack. Carbon dioxide is less hazardous to the fuelcell stack, but it is desirable to limit the concentration of carbondioxide as well, because it will lower the potential in the fuel cellstack, and may be converted to carbon monoxide.

To protect fuel cell stack 14 from being damaged should the productstream contain concentrations of harmful components that are aboveacceptable threshold levels, system 10 further includes a control system50 that monitors the composition of the product stream leaving the fuelprocessor. Control system 50 includes a sensor assembly 52, whichincludes one or more sensors adapted to detect the concentration of aspecific component of the product stream. For example, sensor assembly52 may include a sensor adapted to detect the concentration of carbonmonoxide in the product stream, a sensor to detect the concentration ofcarbon dioxide, etc. It should be understood that assembly 52 mayinclude one or more sensors adapted to detect the concentration of anyselected component or potential impurity in the product stream. Sensorassembly 52 may additionally, or alternatively, measure the compositionof the entire product stream.

Sensor assembly 52 communicates via any suitable communication pathwaywith a controller 54. For example, the sensor may send an electric (i.e.voltage or current) signal to the controller. Other, non-exclusivepathways, include an optical signal, wave form or any other signal thatmay be received by the controller and readily transduced into a controlsignal. Controller 54 compares the measured concentrations to acceptablethreshold values, such as may be stored in a memory portion 56 of thecontrol system. Preferably, the memory portion includes a nonvolatilecomponent in which the threshold values are stored.

It is within the scope of the present invention that the sensor assemblymay not only detect the composition of the product stream, but alsocompare the measured concentration(s) to corresponding stored value(s).In such an embodiment, the sensor assembly signals the controller whenone or more of the threshold concentrations are exceeded.

When any of the threshold concentrations are exceeded, controller 54automatically isolates the fuel cell stack to prevent the contaminatedproduct stream from reaching the stack. This isolation of the fuel cellstack may be implemented in any suitable way. For example, in FIGS. 1and 2, an isolation valve 58 is shown and, when actuated by controlsystem 50, prevents the product stream from reaching the fuel cellstack. As shown, valve 58 diverts the product stream to a waste stream60. System 10 may be described as including an isolation assembly thatincludes any suitable mechanism for preventing, upon actuation, flow ofthe product hydrogen stream to the fuel cell stack. For example, theassembly may include one or more isolation valves.

Sensor assembly 52 should measure the concentration of the productstream sufficiently upstream from isolation valve 58 so that there issufficient time to measure the composition of the stream, determinewhether the stream is contaminated above acceptable levels and thenisolate the fuel cell stack before the stream is introduced to thestack. Therefore, it is preferable for sensor assembly 52 to analyze theproduct stream as far upstream from the isolation valve as possible.

With the embodiment of the fuel processing system shown in FIG. 1,control system 50 may measure the concentration of carbon monoxide orcarbon dioxide in product stream 34. The concentration of carbonmonoxide should be less than 10 ppm (parts per million) to prevent thecontrol system from isolating the fuel cell stack. Preferably, thesystem limits the concentration of carbon monoxide to less than 5 ppm,and even more preferably, to less than 1 ppm. The concentration ofcarbon dioxide may be greater than that of carbon monoxide. For example,concentrations of less than 25% carbon dioxide may be acceptable.Preferably, the concentration is less than 10%, even more preferably,less than 1%. Especially preferred concentrations are less than 50 ppm.It should be understood that the acceptable minimum concentrationspresented herein are illustrative examples, and that concentrationsother than those presented herein may be used and are within the scopeof the present invention. For example, particular users or manufacturersmay require minimum or maximum concentration levels or ranges that aredifferent than those identified herein.

When the fuel processor includes a polishing region, such as the fuelprocessor shown in FIG. 2, other concentrations may be measured insteadof, or in addition to, those described above. For example, becausecarbon dioxide and carbon monoxide are converted into methane and waterin the methanation portion of the polishing region, the concentration ofmethane or water may be monitored. Acceptable concentrations of methanein product stream 42 are less than 1%. Preferably, the concentration ofmethane is less than 1000 ppm, and even more preferably, less than 300ppm. Acceptable concentrations of water in product stream 42 are lessthan 5000 ppm. Preferably, the concentration of water is less than 1000ppm, even more preferably less than 300 ppm.

It should be understood that not all of the compositions being measuredare necessarily harmful to the fuel cell stack. For example, neithermethane nor water will damage the fuel cell stack. The concentrations ofthese compositions may be measured, however, because they are indicativeof a failure in the separation region of the fuel processor. Because thepolishing region acts as an initial safeguard to remove, within itscapacity, carbon dioxide and carbon monoxide from the product stream,detecting the products from the polishing region provides advancedetection of a failure in the separation region. For example,concentrations of methane or water may be detected that exceed theacceptable threshold levels well in advance of the concentrations ofcarbon dioxide or carbon monoxide exceeding the determined maximumlevels. Because of this, detecting methane or water provides earlierdetection of a failure than detecting carbon monoxide or carbon dioxide.

Water provides even earlier detection than methane because it isproduced in stoichiometrically greater quantities than methane. Also,detecting water may be preferred because water/humidity sensors arecurrently less expensive and less prone to interference from othercomponents of the product stream. Of course, as discussed above, it ispreferable that the system detect more than one composition to ensuredetection before the fuel cell stack is poisoned. It may similarly bedesirable for sensor assembly 52 to include redundant sensors for anyselected composition in case one of the sensors is damaged orinoperative.

Monitoring carbon dioxide should also enable earlier detection thanmonitoring carbon monoxide because the relative concentration of carbondioxide in the product stream will increase before that of carbonmonoxide. This is because carbon monoxide is more reactive than carbondioxide, and therefore will be converted into methane and water in thepurification region more readily than carbon dioxide.

It is within the scope of the present invention that control system 50,including sensor assembly 52, may be adapted to detect and isolate thefuel cell stack responsive to concentrations of compositions (elements,compounds, ions, etc.) not discussed herein. So long as a suitablesensor is available to detect the desired composition, controller 54 maystore an associated threshold concentration value for that compositionand automatically isolate the fuel cell stack should the threshold valuebe exceeded.

The above-described method for preventing damage of fuel cell stack 14by early detection of a failure in fuel processor 12 is schematicallyillustrated at 62 in FIG. 3. At 64, the composition of the productstream is measured. As discussed, this may include measuring thecomposition of the entire stream, or detecting the concentration ofselected compositions in the stream. At 66, the measured concentrationor concentrations are compared to stored threshold values. Thesethreshold values correspond to acceptable threshold concentrations ofthe measured compositions, and if exceeded, the fuel cell stack isisolated at 68. If none of the threshold values are exceeded, then themonitoring of the product stream is repeated, thereby providingperiodic, and preferably continuous, monitoring of the product stream.

In addition to isolating the fuel cell stack, control system 50 mayotherwise control the operation of the fuel processing system responsiveto the detected failure in the fuel processor. For example, the productstream, now in the form of waste stream 60, should be utilized orotherwise disposed of. For example, the stream may be vented to theatmosphere. However, it may be desirable to utilize stream 60 for otherpurposes. Because it is no longer suitable for use as a feed for a fuelcell stack 14 does not mean that it is devoid of value. For example,stream 60 may be used as a fuel for a combustion unit to provide heatingto fuel processing system 10 or another device. This combustion mayoccur at the time the fuel cell stack is isolated, or the stream may bestored for future use. It may also be stored for future use other thanfor use as a fuel for a combustion unit.

Control system 50 may also automatically stop additional feedstock frombeing delivered to fuel processor 12. Since the actuation of the controlsystem has occurred, thereby signaling a failure within the fuelprocessor, it follows that there is no need to expend any additionalfeedstock until the failure is fixed. Because this will typicallyinvolve shutting down the fuel processor, the control system may alsoautomatically cause the fuel processor to begin its shut down sequence.Because the fuel cell stack has been isolated, and therefore is nolonger receiving a stream of hydrogen from the fuel processor, the loadbeing applied to the fuel cell stack should also be controlled so thatthe stack's ability to meet the load is not exceeded. Control system 50may automatically trigger this control of the applied load. In additionto the above safety steps, control system 50 may also actuate a responsesystem, which may include an alarm or other suitable device to alertusers that there has been, or imminently may be, a failure within thefuel processor and that the fuel cell stack is no longer receiving ahydrogen stream from the fuel processor.

The above-described steps of the invented method, which may beimplemented by the control system, are shown in FIG. 3 at 70-78. None ofthese steps are essential, however, it may be preferable to implementany or all of these steps in a system or method according to the presentinvention.

Control system 50 may be implemented with either a digital or an analogcircuit, or a combination of the two. For example, the controller mayinclude software executing on a processor, or it may be an analogcircuit. Regardless of how controller 54 is implemented, it may, butdoes not necessarily, include a user interface. An example of a userinterface is schematically shown in FIG. 4 and indicated generally at80. Interface 80 enables a user to monitor and/or interact with theoperation of the controller.

As shown, interface 80 includes a display region 82 in which informationis presented to the user. For example, display region 82 may display thecurrent values measured by sensor assembly 52. As discussed, this mayinclude the entire composition of the product stream, or concentrationsof selected components thereof. Other information regarding theoperation and performance of the fuel processing system may also bedisplayed in region 82. Also shown in FIG. 4 is a user-signaling device84 that alerts a user when an acceptable threshold level has beenexceeded and the fuel cell stack has been isolated. Device 84 mayinclude an alarm, lights, or any other suitable mechanism or mechanismsfor alerting users.

User interface 80 may also include a user input device 86 through whicha user communicates with the control system. For example, input device86 may enable a user to adjust the threshold concentration values and/orto select the particular composition or compositions to be detected.Input device 86 may include any suitable device for receiving userinputs, including rotary dials and switches, push-buttons, keypads,keyboards, a mouse, touch screens, etc.

It should be understood that it is within the scope of the presentinvention that the fuel processing system may include a control systemwithout a user interface, and that it is not required for the userinterface to include all of the elements described herein. The elementsdescribed above have been schematically illustrated in FIG. 4collectively, however, it is within the scope of the present inventionthat they may be implemented separately. For example, the user interfacemay include multiple display regions, each adapted to display one ormore of the types of user information described above. Similarly, asingle user input device may be used, and the input device may include adisplay that prompts the user to enter requested values or enables theuser to toggle between input screens.

While the invention has been disclosed in its preferred form, thespecific embodiments thereof as disclosed and illustrated herein are notto be considered in a limiting sense as numerous variations arepossible. It is intended that any singular terms used herein do notpreclude the use of more than one of that element, and that embodimentsutilizing more than one of any particular element are within the spiritand scope of the present invention. Applicants regard the subject matterof the invention to include all novel and non-obvious combinations andsubcombinations of the various elements, features, functions and/orproperties disclosed herein. No single feature, function, element orproperty of the disclosed embodiments is essential to all embodiments.The following claims define certain combinations and subcombinationsthat are regarded as novel and non-obvious. Other combinations andsubcombinations of features, functions, elements and/or properties maybe claimed through amendment of the present claims or presentation ofnew claims in this or a related application. Such claims, whether theyare broader, narrower or equal in scope to the original claims, are alsoregarded as included within the subject matter of applicants' invention.

1. A method for preventing contamination of a fuel cell stack in a fuelprocessing system that includes a fuel processor adapted to produce aproduct hydrogen stream and a fuel cell stack adapted to receive theproduct hydrogen stream, the method comprising: measuring theconcentration of at least one component of the product hydrogen stream;comparing the measured concentration to a corresponding threshold value,wherein the at least one component includes a selected component that,if present in the product hydrogen stream in a concentration that is atleast as great as the corresponding threshold value, is not harmful tothe fuel cell stack; and preventing delivery of the product hydrogenstream to the fuel cell stack if the measured concentration exceeds thecorresponding threshold value.
 2. The method of claim 1, wherein themethod further includes automatically preventing delivery of the producthydrogen stream to the fuel cell stack if the measured concentrationexceeds the corresponding threshold value.
 3. The method of claim 1,wherein the preventing step includes actuating an isolation valveconfigured to interrupt the flow of the product hydrogen stream to thefuel cell stack if the measured concentration exceeds the correspondingthreshold value.
 4. The method of claim 3, wherein the preventing stepincludes automatically actuating the isolation valve if the measuredconcentration exceeds the corresponding threshold value.
 5. The methodof claim 1, wherein the preventing step includes venting the producthydrogen stream if the measured concentration exceeds the correspondingthreshold value.
 6. The method of claim 5, wherein the preventing stepincludes automatically venting the product hydrogen stream if themeasured concentration exceeds the corresponding threshold value.
 7. Themethod of claim 1, wherein the preventing step includes storing theproduct hydrogen stream if the measured concentration exceeds thecorresponding threshold value.
 8. The method of claim 7, wherein thepreventing step includes automatically storing the product hydrogenstream if the measured concentration exceeds the corresponding thresholdvalue.
 9. The method of claim 1, wherein the method further includesshutting down the fuel processor if the measured concentration exceedsthe corresponding threshold value.
 10. The method of claim 9, whereinthe method further includes automatically shutting down the fuelprocessor if the measured concentration exceeds the correspondingthreshold value.
 11. The method of claim 1, wherein the method furtherincludes actuating a response system if the measured concentrationexceeds the corresponding threshold value.
 12. The method of claim 11,wherein the method further includes automatically actuating the responsesystem if the measured concentration exceeds the corresponding thresholdvalue.
 13. The method of claim 1 1, wherein the response system includesa user-notification device.
 14. The method of claim 1, wherein themethod further includes limiting a load that may be applied to the fuelcell stack if the measured concentration exceeds the correspondingthreshold value.
 15. The method of claim 14, wherein the method furtherincludes automatically limiting the load that may be applied to the fuelcell stack if the measured concentration exceeds the correspondingthreshold value.
 16. The method of claim 1, wherein the selectedcomponent includes methane.
 17. The method of claim 16, wherein thecorresponding threshold value corresponds to methane comprising 1% ofthe product hydrogen stream.
 18. The method of claim 16, wherein thecorresponding threshold value corresponds to 1000 ppm of methane in theproduct hydrogen stream.
 19. The method of claim 16, wherein thecorresponding threshold value corresponds to 300 ppm of methane in theproduct hydrogen stream.
 20. The method of claim 1, wherein the selectedcomponent includes carbon dioxide.
 21. The method of claim 20, whereinthe corresponding threshold value corresponds to carbon dioxidecomprising 25% of the product hydrogen stream.
 22. The method of claim20, wherein the corresponding threshold value corresponds to carbondioxide comprising 10% of the product hydrogen stream.
 23. The method ofclaim 20, wherein the corresponding threshold value corresponds tocarbon dioxide comprising 1% of the product hydrogen stream.
 24. Themethod of claim 20, wherein the corresponding threshold valuecorresponds to 50 ppm of carbon dioxide in the product hydrogen stream.25. The method of claim 1, wherein the selected component includeswater.
 26. The method of claim 25, wherein the corresponding thresholdvalue corresponds to 5000 ppm of water in the product hydrogen stream.27. The method of claim 25, wherein the corresponding threshold valuecorresponds to 1000 ppm of water in the product hydrogen stream.
 28. Themethod of claim 25, wherein the corresponding threshold valuecorresponds to 300 ppm of water in the product hydrogen stream.
 29. Themethod of claim 25, wherein the selected component further includesmethane.
 30. The method of claim 25, wherein the selected componentfurther includes carbon dioxide.